A single carriageway (British English) or undivided highway (American English) is a road with one, two or more lanes arranged within a single carriageway with no central reservation to separate opposing flows of traffic.
A two-lane road or two-lane highway is a single carriageway with one lane for each direction. A single-track road has a single lane with passing places for traffic in both directions. Road traffic safety is generally worse for high-speed single carriageways than for dual carriageways due to the lack of separation between traffic moving in opposing directions.
The term single carriageway is used for roads in the Republic of Ireland. Speed limits on single-carriageway roads vary depending on their classification: national primary roads and national secondary roads have a general speed limit of 100 km/h (62 mph), while regional roads and local roads have a general speed limit of 80 km/h (50 mph). In urban areas, the general speed limit is 50 km/h (31 mph).
The maximum UK speed limit for single-carriageway roads is lower than the maximum for dual-carriageway roads. The National Speed Limit, which is lower for built-up areas, only applies in places where a lower numeric speed limit is not in place. The UK has one major single-carriageway motorway, the A38(M), but a number of link roads at motorway interchanges are single-carriageway.
In the U.S. the expression "undivided highway" is likely to mean a multi-lane road with only striping (paint) (but no median) between the two directions of traffic flow. A road with two lanes of traffic moving in opposite directions is specifically called a two-lane road.
In keeping with the U.S. Department of Transportation's Manual on Uniform Traffic Control Devices (MUTCD), since the early 1970s all numbered highways in the U.S. are striped by color to show the direction of traffic flow. Two-way undivided roads have an amber center line (and, if there are baseline shoulder stripes, they are solid white on both sides). This center line may be solid, broken, or a combination of the two, with the different styles denoting whether passing (which requires a driver to move into the lane used by oncoming traffic) is permitted at a given location.
Multilane roads use broken white lines between lanes moving in the same direction; at least one solid amber line lies to the left of the lane which borders traffic moving in the opposite direction, and the right sideline is solid white. Drivers can always tell the direction of traffic flow by looking at the striping coloration.
Since successful experiments in the late 1960s, some urban undivided highways in the U.S. have had a "neutral" (used by both directions of flow) left-turn lane. This configuration is called a two-way left turn lane (TWLTL, often pronounced "twittle"). Essentially, this configuration puts a turning lane in the position of where a median would be if the road were divided.
These roads almost always have an odd number of lanes overall, usually five (two lanes in each direction with a middle turning lane used by both directions of flow), but three-lane and seven-lane versions are not uncommon. TWLTLs are most frequently built in suburban commercial areas where there are a large number of closely spaced driveways (or minor streets).